Chemical vapor deposition wafer boat

ABSTRACT

A chemical vapor deposition wafer boat for supporting a plurality of wafers in an evenly spaced, upright orientation perpendicular to the axis of the boat comprises a cylinder having closed ends and comprised of mutually engaging upper and lower hemicylinders. The upper hemicylinder has diffusion zones with gas flow passageways therein in the ends and zones within from 0 to 75 and within from 0 to 15 degrees from a vertical plane through the cylinder axis. The remainder of the hemicylinder wall and the ends are baffle areas without gas flow passageways. The ends and sidewall of the lower hemicylinder comprise gas diffusion zones. The gas flow passageways comprise from 0.5 to 80 percent of the surface area of the respective gas diffusion zones.

This application is a continuation of copending application Ser. No.607,065 filed May 4, 1984, now U.S. Pat. No. 4,582,020.

FIELD OF THE INVENTION

This invention relates to a chemical vapor deposition wafer boat. Inparticular, this invention relates to a wafer boat for use with chemicalvapor deposition processes in vertical devices to produce highlyuniform, uncontaminated coatings of silicon dioxide and other materialson substrates.

BACKGROUND OF THE INVENTION

Chemical vapor deposition (CVD) is the process of depositing a solidmaterial from a gaseous phase onto a substrate by means of a chemicalreaction. This deposition reaction is generally thermal decomposition,chemical oxidation, or chemical reduction. In one example of thermaldecomposition, organometallic compounds are transported to the substratesurface as a vapor and are reduced to the elemental metal state on thesubstrate surface. For chemical reduction, the reducing agent mostusually employed is hydrogen, although metal vapors can also be used.The substrate can also act as a reductant as in the case of tungstenhexafluoride reduction by silicon. The substrate can also supply oneelement of a compound or alloy deposit. The CVD process can be used todeposit many elements and alloys as well as compounds including oxides,nitrides and carbides.

In the present invention, CVD technology is used to manufacture highlyuniform silicon dioxide deposits on semiconductor wafer substrates.

Chemical vapor deposition of electronic materials is described by T. L.Chu et al, J. Vac. Sci. Technol. 10:1 (1973) and B. E. Watts, Thin SolidFilms. 18:1 (1973). They describe the formation and doping of epitaxialfilms of such materials as silicon, germanium and GaAs, for example. Asummary of the chemical vapor deposition field is provided by W. A.Bryant, "The Fundamentals of Chemical Vapour Deposition", Journal ofMaterials Science. 12:1285 (1977). Low pressure CVD production ofsilicon dioxide deposits is summarized by R. Rosler, Solid StateTechnology. 63-70 (April 1977), the contents thereof being incorporatedby reference.

DESCRIPTION OF THE PRIOR ART

The positioning of a plurality of wafers in a row in a vapor depositiondevice has been previously described in U.S. Pat. No. 3,471,326, forexample, and placing them in a vertical orientation has been describedin U.S. Pat. Nos. 3,922,467 and 4,018,183. Open wafer support boatswhich can be preloaded before insertion into tube furnaces are describedin U.S. Pat. Nos. 4,220,116 and 4,355,974, and similar boats combinedwith shrouds of parallel rod gas flow turbulence producers are describedin U.S. Pat. Nos. 4,203,387 and 4,309,240.

A boat comprising a perforated hemicylindrical upper section and a lowerhemicylindrical lower section of axially parallel rods is disclosed inU.S. Pat. No. 4,256,053. Boats comprising perforated cylindricalcylinders with end closures are described in U.S. Pat. No. 4,098,923 and4,232,063. These boats are designed to be preloaded and placed inconventional tube furnaces, creating non-laminar mixing turbulence inthe gas flowing into the wafer section. Because of the nature of the gasflow patterns with these devices, it is necessary for all surfaces to beuniformly open for gas inflow, exposing the wafer surfaces tounavoidable jets or streams of reactive gas and particulatecontamination. In vertical reactors such as those described in commonlyassigned, copending application Ser. No. 528,193 filed Aug. 31, 1983,these prior art boats are unsuitable for providing uniform films ofsilicon dioxide and other materials, both because of their inability toprovide the requisite gas diffusion flow and for their failure roexclude particulates from the wafer surface.

SUMMARY OF THE INVENTION

The chemical vapor deposition wafer boats of this invention are meansfor supporting a plurality of wafers in an evenly spaced, uprightorientation perpendicular to the axis of the boat. The boat comprises acylinder having closed ends and having mutually engaging upper and lowerhemicylinders, the upper hemicylinder having diffusion zones with gasflow passageways therein in the ends and in zones within from 0 to 75degrees from a horizontal plane through the cylinder axis and from 0 to15 degrees from a vertical plane through the cylinder axis. Theremainder of the hemicylinder wall constitutes baffle areas without gasflow passageways. The ends and sidewall of the lower hemicylinderconstitute gas diffusion zones. The gas flow passageways comprising from0.5 to 80 percent of the surface area of the respective gas diffusionzones.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of the cylindrical wafer boat of this invention.

FIG. 2 is a cross-sectional view of the cylindrical wafer boat of thisinvention taken along the line 2--2 in FIG. 1 with wafers omitted.

FIG. 3 is a fragmentary cross-sectional view of the lower wafer supportrails taken along line 3--3 in FIG. 2.

FIG. 4 is a fragmentary cross-sectional view of the upper wafer supportrails taken along line 4--4 in FIG. 2.

FIG. 5 is a cross-sectional representational view of the reactionchamber of the vertical CVD device showing the gas flow patterns aroundand in the wafer boats.

FIG. 6 is a fragmentary cross-sectional view of a wafer boat with wafersloaded in the front-to-back position according to this invention.

FIG. 7 is a fragmentary cross-sectional view of a wafer boat with wafersloaded in the back-to-back position according to this invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a side view of the cylindrical wafer boat of this invention,and FIG. 2 is a cross-sectional view taken along the line 2--2 inFIG. 1. The central axis of the cylindrical wafer boat 2 is horizontal,and wafers supported therein for coating are supported in an uprightorientation. The term "upright", as used herein, denotes that the wafersare supported on their edges, and the planes of the wafer surfaces aresubstantially vertical, that is, within 10° of vertical. The optimumorientation is a uniform slight tilt from the vertical of less than 5°.

The inner surfaces of the cylindrical walls have the shape of andconform to the outer edges of the individual wafers to be coated, beingprecisely spaced from the edges of the wafers. The cylindrical waferboat 2 comprises an upper hemicylindrical section 4 and a lowerhemicylindrical section 6 having mutually engaging opposed surfaceswhich join in a horizontal plane approximately through the central axisof the cylinder 2. The ends 8 and 10 of the upper hemicylinder and 12and 14 of the lower hemicylinder are closed with gas flow passagewaystherein. Leg projections 16 and 18 project from the lower surface ofhemicylinder 6 and are preferably integral therewith. The lower surface20 of the hemicylinder 6 can be a flat section to engage the flat lowerindexing edge typically present on a wafer, if desired. The legprojections 16 and 18 maintain the wafer boat in a stable orientation,precisely positioned in the reaction zone with the lower surface thereofat least 0.1 inches and preferably at least 0.3 inches above thesupporting surface.

Referring to FIG. 2, lower gas flow passageways 22 and upper gas flowpassageways 24 are present in the sidewall portions of the upperhemicylinder which constitute the diffusion zones thereof. The ends 8and 10 are also diffusion zones gas flow passageways 23 being positionedin the end walls 8 and 10. The lower passageways 22 are within thediffusion Zones C corresponding to Angle C in FIG. 2. Zones Bcorresponding to Angle B is closed and free of passageways. Angle C isfrom 10 to 75° and preferably from 10° to 60° of the horizontal planedividing the cylinder into upper and lower hemicylinders. Additionalopenings 24 can be provided in the upper hemicylinder within Zone Acorresponding to Angle A. Angle A is within 15° and preferably within10° of the vertical plane through the axis of the cylinder 2.Passageways 22 and 24 can be circular holes, as shown, or have oval,elliptical, rectangular, slotted or other cross-sectional shapes, ifdesired. In one embodiment, the passageways 22 are positioned throughoutthe diffusion Zone C in a substantially even distribution. Thecross-sectional area of the gas flow passageways can be from 0.5 to 80percent, preferably from 0.5 to 40, and optimally from 0.5 to 20 percentof the total outer surface area of Zone C (including the portionsoccupied by holes). The upper and lower hemicylinders are preferablysymmetrical about the vertical axis, and the Zones B and C are presenton both sides of the vertical axis of the upper hemicylinder 4 in asymmetrical configuration.

The entire lower hemicylinder wall 6 is a gas diffusion zone andpreferably has gas flow passageways 26. These are preferably uniformlydistributed and can have the shapes described above with respect to thepassageways in the upper hemicylinder 4. The cross-sectional area of thegas flow passageways 26 can be from 0.5 to 80 percent, preferably from0.5 to 40, and optimally from 0.5 to 20 percent of the total outersurface area of the lower hemicylinder 6 (including the portionsoccupied by holes). The ends 12 and 14 also have gas flow passagewaysarranged to permit gas flow into the ends of the cylinder 2. In endclosure 14, for example, the passageway 27 is an open slot having anarcuate shape adjacent the wall 6 conforming to the shape of thesidewall 6 and bottom wall 20. The passageways 27 in the ends 12 and 14preferably occupy less than 20 percent of the end closure area.

The legs 16, 17 and 18 are supported by a cross-beam 28 preferablyintegral therewith. The legs 16, 17 and 18 are designed to support thecylindrical wafer boat 2 in a stable orientation during the coatingoperation. They are also designed to straddle and engage a loading forkprojection (not shown) of a loading apparatus, by which means the boatscan be automatically and rapidly loaded and unloaded from the wafersupport surface. Such a loading apparatus is described in commonlyassigned, copending application Ser. No. 529,415 filed Sept. 6, 1983.

The rails 30, 32, 34 and 36 maintain the wafers placed in the boat in aprecisely spaced, upright position.

FIG. 3 is a fragmentary cross-sectional view of the wafer boat takenalong line 3--3 in FIG. 2 showing details of the lower wafer supportrails . The slots 38 in rail 34 have angularly sloped sides 40 taperingto merge with the bottom surfaces 42, maintaining the bottom of thewafers placed therein in a precisely determined spacing but leaving thewafer surface fully exposed

FIG. 4 is a fragmentary cross-sectional view of the wafer boat takenalong line 4--4 in FIG. 2 showing details of the upper wafer supportrails. The slots 44 in rail 36 support the wafers placed therein in thevertical orientation. They have tapered portions 46 and 48 whichfacilitate loading and reduce the portion of the wafer surface masked bythe slots when the wafers are bottomed against the slot surface 50.

FIG. 5 is a cross-sectional representational view of the reactionchamber of the vertical CVD device showing the gas flow patterns aroundand in the wafer boats with the wafers omitted for a clearerrepresentation. The gas exits from the outlet 60 of the gas distributor62 into the reaction chamber defined by the cover 64 and the supportplate 66. Gas flows through the reaction chamber around the wafer boats68 and 70, exiting through gas outlet openings 73 and 74 in the supportplate 66. The support plate together with the lower bowl shaped element76 define a gas removal system under reduced pressure. The gas flowscontinuously during the coating operation from the outlet 60 through thereaction chamber and out through the gas removal system.

The wafer surfaces are protected by the boat configuration from directimpingement by turbulent gas flows, or jets or streams of gas flowing inpaths. Gas flow into the boat interior is by diffusion. Zone Brepresented by 72 does not have passageways and provides a bafflesurface which shields the wafers from the turbulence surrounding the gasoutlet 60, eliminating a major source of the commonplace surfaceirregularities characteristic of the previously known devices. The gasdiffuses into the boat interior through the passageways 22, 23, 24, 26and 27 (FIG. 2).

In the preferred embodiments of this invention, wafer spacing, the gapbetween wafers and the passageway walls, the percentage open arearepresented by the gas flow passageways, and boat surface roughness arecontrolled to provide optimum conditions which yield improvedwafer-to-wafer and wafer edge-to-center coating uniformity.

FIG. 6 is a fragmentary cross-sectional view of a wafer boat with wafersloaded in the front-to-back position according to this invention. Theend 80 is precisely spaced from the wafers 84 at a distance "a" which isat least 2 mm. and preferably is from 2 to 4 mm, sufficient to permitgas flow passageways 85 between the end wafer and the hemicylinder end80. The front surfaces 86 are the coating surfaces and are facing to theright in this embodiment. The inlet passageways 88 are adjacent the backsurfaces 90 of the wafers 84. The distance "b" between the outer edgesof the wafers 84 and the inner surface 92 of the hemicylinder 82 can befrom 1.0 to 5.0 mm.

The end 94 of the lower hemicylinder and the inner surface 96 thereofare correspondingly spaced (dimensions "a" and "b") from the outer edgesof the wafers 84. The gas inlet passageways 98 in the lower hemicylinderwall 99 are spaced adjacent the back surfaces 90 of the wafers and at amaximum distance from the front wafer surfaces 86. The wafers 84 rest inslots 100 in rails 102 on the bottom of the lower hemicylinder, and aremaintained in a predetermined wafer-to-wafer spacing by slots 104 in thesiderails 106. The wafer-to-wafer spacing "c" corresponds to thedistance "d" between slots 104 and can be from 2.5 to 12.5 mm.

FIG. 7 is a fragmentary cross-sectional view of a wafer boat with wafersloaded in the back-to-back position according to this invention. The end110 is precisely spaced from the end wafer 114 at a distance "e" whichis at least 2 mm. and is preferably from 2 to 4 mm. The front wafercoating surfaces 116 and 118 of the wafers 115 are opposed(back-to-back) in this embodiment. The inlet passageways 120 aremaximally spaced from the coating surfaces 116 and 118 of the wafers115. The distance "f" between the outer edges of the wafers 115 and theinner surface 122 of the hemicylinder 112 can be from 1.0 to 5.0 mm.

The end 124 of the lower hemicylinder and the inner surface 126 thereofare also spaced from the outer edges of the wafers 115 by the distances"e" and "f" described in conjunction with the upper hemicylinder. Thegas ihlet passageways 127 in the hemicylinder wall are spaced at amaximum distance from the coating surfaces 116 and 118 of the wafers115. The wafers 115 rest in slots 130 in rails 132 on the bottom of thelower hemicylinder, and are maintained in a predetermined wafer-to-waferspacing by slots 134 in the siderails 136. The wafer-to-wafer spacing"g" corresponds closely to the distance "h" between slots 134 and can befrom 2.5 to 12.5 mm.

The boat surface is preferably roughened and has a roughnesscorresponding to a surface area ratio of roughened boat surface area tothe corresponding smooth boat surface area of from 1 to 4.

Preferred and optimum dimensions for wafer boats to be used in verticalCVD devices having gas flow patterns such as are shown in FIG. 5 areshown in Table A.

                  TABLE A                                                         ______________________________________                                                   Preferred Ranges                                                                          Optimum Range                                          ______________________________________                                        Wafer spacing, mm.                                                                         5-8       8-11     9-10                                          Roughness.sup.m                                                                            1.0-3.0  1.0-3.0  1.5-2.5                                        Gap, wafer edge to                                                                         2.0-3.0  2.0-3.0  2.0-3.0                                        Boat surface, mm.                                                             Passageway area.sup.n                                                                      0.5-2.5  1.0-6.0  2.0-4.0                                        ______________________________________                                         .sup.m ratio of roughened boat surface area to smooth boat surface area       .sup.n percentage of wall surface occupied by gas flow passageways       

As can be seen from Table A, if wafer spacing is reduced to increasewafer throughput, the area of the gas flow passageway is also reducedfor optimum uniformity. The back-to-back loading configuration shown inFIG. 7 provides a maximum spacing for a corresponding load of wafers,and is the preferred arrangement for maximum loading at-optimum coatingconditions.

The controlled gas diffusion effects edge to center and wafer to wafercoating variations of less than 2 percent and under most carefullycontrolled optimum conditions of less than one percent, making theprocess particularly suitable for manufacturing VLSI devices. The waferboats of this invention are particularly suitable for use in thevertical CVD apparatus described in commonly assigned, copendingapplication Ser. No. 528,193 filed Aug. 31, 1983, the entire contents ofwhich are hereby incorporated by reference.

The invention claimed is:
 1. A chemical vapor deposition wafer boat means for supporting a plurality of wafers in an evenly spaced, upright orientation substantially perpendicular to the axis of the boat during loading and chemical vapor deposition in a reaction chamber and for protecting the wafers from direct impingement of reaction gas streams entering the reaction chamber, the boat comprising a cylinder having closed ends and comprised of mutually engaging upper and lower hemicylinders, the upper hemicylinder having diffusion zones with gas flow passageways therein in the ends and in zones within from 10° to 60° from a horizontal plane through the cylinder axis and from 0° to 15° from a vertical plane through the cylinder axis, the remainder of the hemicylinder wall being baffle means without gas flow passageways for preventing direct impingement of gas streams on wafers in the boat, the ends and sidewall of the lower hemicylinder comprising gas diffusion zones, the gas flow passageways comprising from 0.5 to 40 percent of the surface area of the respective gas diffusion zones.
 2. The chemical vapor deposition wafer boat means of claim 1 wherein the upper hemicylinder wall has diffusion zones with gas flow passageways therein in zones within from 0° to 10° from the vertical plane through the cylinder axis.
 3. The chemical vapor deposition wafer boat means of claim 1 wherein the boat has a plurality of inner slot means for positioning wafers in the boat, the slot means comprising individual slots for each wafer, the distance between adjacent slots being within the range of from 2.5 to 12.5 mm.
 4. The chemical vapor deposition wafer boat means of claim 1 wherein the boat has a plurality of inner slot means for positioning wafers in the boat, the slot means comprising pairs of closely adjacent slots for wafer spacing in a back-to-back configuration, the distance between adjacent pairs of slots being within the range of from 2.5 to 12.5 mm.
 5. The chemical vapor deposition wafer boat means of claim 1 wherein the inner dimensions of the upper and lower hemicylinders is selected to provide a gap between wafer edge and inner cylinder surface of a loaded boat within the range of from 1.0 to 5.0 mm.
 6. A chemical vapor deposition apparatus comprising a vapor deposition chamber having a gas inlet means positioned in the upper portion of the chamber for introducing gases into the chamber and a gas outlet means positioned in the lower portion of the chamber for removing gases from the chamber, a cylindrical wafer boat means positioned between the gas inlet means and the gas outlet means for supporting a plurality of wafers in an evenly spaced, upright orientation, the boat means comprising a cylinder having closed ends and comprised of mutually engaging upper and lower hemicylinders, the upper hemicylinder having diffusion zones with gas flow passageways therein in the ends and in zones within from 10° to 60° from a horizontal plane through the cylinder axis and from 0° to 15° from a vertical plane through the cylinder axis, the remainder of the hemicylinder wall being baffle means without gas flow passageways positioned between the gas inlet means and interior of the boat means for shielding wafers in the wafer boat from direct impingement of gases from the gas inlet means, the ends and sidewall of the lower hemicylinder comprising gas diffusion zones with gas flow passageways therein, the gas flow passageways comprising from 0.5 to 40 percent of the surface area of the respective gas diffusion zones.
 7. The chemical vapor deposition apparatus of claim 1 wherein the upper hemicylinder wall of the baot means has diffusion zones with gas flow passageways therein in zones within from 0° to 10° from the vertical plane through the cylinder axis.
 8. The chemical vapor deposition apparatus of claim 6 wherein the boat means has a plurality of inner slot means for positioning the wafers in the boat, the slot means comprising individual slots for each wafer, the distance between adjacent slots being within the range of from 2.5 to 12.5 mm.
 9. The chemical vapor deposition apparatus of claim 6 wherein the boat means has a plurality of inner slot means for positioning wafers in the boat, the slot means comprising pairs of closely adjacent slots for wafer spacing in a back-to-back configuration, the distance between adjacent pairs of slots being within the range of from 2.5 to 12.5 mm.
 10. The chemical vapor deposition apparatus of claim 6 wherein the inner dimensions of the upper and lower hemicylinders is selected to provide a gap between wafer edge and inner cylinder surface of a loaded boat within the range of from 1.0 to 5.0 mm. 